I buy seeds at a discount each fall, and almost always have some left overs from the spring order. They all germinate very well, and I don’t even try to do much of anything to preserve viability.
I just throw them in a zip bag, with the date, and throw them in a card board box on a shelf in the basement where they don’t get wet.
Keeping viable seed from year to year is fairly easy if you keep them in a dry, fairly stable cool temperature, and away from sunlight
A basement generally meets these requirements.
As was stated , just make sure they are stored after they are absolutely dry, or they will mold.
Always conduct a germination test on saved seed, unless recommended only to direct seed;
your garden productivity may depend on it, as frequently crop loss timing does not allow for optimum re-seeding.
Two websites about seed longevity for information:
Life Expectancy of Vegetable Seeds Chart ( Iowa State University)
www.ipm.iastate.edu/ipm/hortnews/1999/4-2-1999/veggielife.html
Seedman.com (info on storage,germination, # of seeds per ounce, and chart of seed longevity)
www.seedman.com/veggerm.htm